1. Field of the Invention
This invention relates generally to semiconductor technology, and more particularly, to maintaining substantially constant temperature of a semiconductor device under test.
2. Discussion of the Related Art
Semiconductor devices typically undergo a variety of electrical test procedures, including short-circuit tests, burn-in tests, and functional device tests to ensure their proper operation. During for example functional testing, it is important that the temperature of the device under test be held at the chosen, substantially constant value. However, during such functional testing, the power level of the device may vary greatly, causing the temperature of the device to fluctuate. In dealing with this problem, it is known to provide a thermal head assembly 20 a surface 22 of which may be brought into contact with the lid 24 of the device under test 26, for example, a flip-chip mounted on a printed circuit board 28 (FIG. 1). The thermal head assembly 20 includes an electrical heating element 30 in the form of a resistor, the output of which can be increased and decreased by respectively increasing and decreasing electrical current flow therethrough, and a passage 32 through which coolant 34, for example, water, may flow. By changing electrical current flow and/or providing or cutting off coolant flow, the temperature of the thermal head assembly 20, and thus the temperature of the device under test 26 adjacent thereto, can be adjusted or varied. As the temperature of the device under test 26 varies due to changes in power level thereof as described above, the temperature of the thermal head assembly 20 is caused to change to compensate for the changing temperature of the device 26, in order to attempt to maintain the device under test 26 at a constant, chosen temperature.
As pointed out above, changes in power level of the device under test 26 cause the device temperature to change. Changes in power level of the thermal head assembly 20 also cause the device under test 26 temperature to change. The temperature of the device under test 26 can be kept substantially constant if the total power dissipated by the thermal head assembly 20 and device under test 26 can be held substatially constant. Since the device under test 26 runs through a known or substantially known testing sequence, the individual levels of power dissipated by the device 26 over time during this testing sequence can be known. Based on this information, the thermal head assembly 20 can be caused to have individual levels of power dissipated thereby over time so that during the time period of the testing sequence, the total of the power dissipated by the thermal head assembly 20 plus the power dissipated by the device under test 26 remains substantially constant.
Since the heating element 30 has a known resistive value, it would appear that a certain power level to be dissipated by the thermal head assembly 20 can be readily achieved by providing a chosen current or chosen voltage to the heating element 30. However, some pure metals used as heating elements have a thermal coefficient of electrical resistance of about 1/300 per xc2x0 C. at approximately room temperature. If the temperature of a heating element 30 made of such material increases from for example 20xc2x0 C. (approximate room temperature) to 95xc2x0 C., the resistance of the heating element 30 can change approximately 25 percent. It will readily be seen that if a chosen current is applied to such a heating element 30 with such heating element 30 at an elevated temperature, more power will be dissipated than predicted, due to this increased resistance. Conversely, if a chosen voltage is applied to the heating element 30, current therethrough will decrease due to increased resistance, and less power will be dissipated thereby than predicted.
Therefore, what is needed is apparatus for providing that the signal applied to the heating 30 element for determining the power level thereof is adjusted to compensate for a variation in resistance of the heating element 30 due to changes in temperature thereof.
The apparatus shown and described herein provides for a chosen level of power dissipation of a resistor, with the resistance of the resistor changing upon a change in temperature thereof, and uses feedback circuitry to correct for variations in the resistance of the resistor. The apparatus includes a shunt in series with the resistor, a first differential amplifier, with voltage drop across the resistor being provided to first and second input terminals of the first differential amplifier, and a second differential amplifier, voltage drop across the shunt being provided to first and second input terminals of the second differential amplifier. A voltage multiplier receives signals from the output terminals of the first and second differential amplifiers. An output terminal of the voltage multiplier is connected to an inverting input terminal of a power operational amplifier, and a programming sequence voltage is supplied to a non-inverting input terminal of the power operational amplifier. The output terminal of the power operational amplifier is connected to the resistor for providing voltage thereto.
The present invention is better understood upon consideration of the detailed description below, in conjunction with the accompanying drawings. As will become readily apparent to those skilled in the art from the following description, there is shown and described an embodiment of this invention simply by way of the illustration of the best mode to carry out the invention. As will be realized, the invention is capable of other embodiments and its several details are capable of modifications and various obvious aspects, all without departing from the scope of the invention. Accordingly, the drawings and detailed description will be regarded as illustrative in nature and not as restrictive.